The invention relates to wireless communication systems in general and more particularly to the organization of such systems.
Conventional radio telephone systems such as cellular systems use cell-sites having co-located transmitters and receivers to cover geographical regions referred to as cells. Several cell-sites disposed within a particular geographic area are coupled to a master controller called a mobile telephone switching office (MTSO). The MTSO controls the cell-sites and provides an interface connection to the public switched telephone network (PSTN).
Each conventional cell-site uses pre-assigned channel sets to communicate with mobile units in a service area covered by the cell-site. Each channel set typically includes a pair of carrier frequencies with each carrier frequency being used for respective up-link or down-link communications with a mobile unit. Neighboring cell-sites use different channel sets to avoid interference on the same channel and adjacent channels between adjacent service areas.
Conventional cellular systems provide mobility to a subscriber through a procedure referred to as hand-off. According to this procedure, geographically adjacent cell-sites are considered to be neighbor cell-sites. A neighbor cell-site is the cell-site to which a call can be transferred as a mobile unit traverses a current cell-site boundary. A data table called a neighbor list specifies the cell-sites that can receive a hand-off from a particular cell-site. In addition, to increase the simultaneous communication capacity of a system, channel reuse is employed where two sufficiently distant base stations simultaneously use the same channel.
The channel sets and neighbor lists assigned to particular cell-sites as well as cell-site transmission powers are examples of system organization parameters that define the operating characteristics of the system. Such parameters are typically determined using propagation models prior to installation of a system. After installation, the system coverage area produced by the determined parameter settings is verified by field testing. During a typical field test, a mobile test unit is moved throughout the service area while the base stations and test unit transmit respective test frequencies. As the test unit is moved from one sampling location to the next, the respective signal strength of the test frequencies and the corresponding geographic location is detected at the respective base stations and test unit to verify that the system can provide service to the intended coverage area.
Typical wireless communication systems do not have the capability to automatically identify and adapt parameter settings to such changes. Environmental changes that often require parameters setting adjustments include the construction of structures in the coverage area, such as a building in an outdoor cellular system or a added walls in an indoor system, or the installation of another wireless communication system in close proximity to the current system. Such changes could degrade system performance and often require an installer to perform computer modeling of the coverage area again to determine the proper parameters settings.
A method that possesses the limited ability of determining frequency channels used by respective cell-sites is described in M. Almgren et al., xe2x80x9cAdaptive Channel Allocation in TACSxe2x80x9d, IEEE Global Telecommunication Conference Record, pp. 1517-1521 (1995), which is incorporated by reference herein. According to this method, each cell-site monitors received signal strength (RSS) on respective sets of channels over time and uses the channels for establishing communications that have the lowest interference.
Also, some time division multiple access (TDMA) systems, such as those adhering to the Telecommunication Industry Association Interim Standard 136 (TIA IS-136 standard), have the limited ability to dynamically allocate channels during operation of the systems to achieve greater spectral efficiency and communication capacity. In such systems, a cell-site can request an idle mobile unit to measure the RSS or bit error rate of different communication channels and transmit the measurement information back to the cell-site. Such RSS or bit error rate information indicates the interference on the respective channels. Communication can then be established with the mobile unit using the channel with the lowest interference. However, such a dynamic allocation technique is limited to channel allocation for the respective mobile unit that provides the RSS information.
Thus, a need exists for a radio telephone system having enhanced spectral efficiency that employs a substantially automated determination of system organization parameters and that is capable of adjusting for environmental changes.
The invention provides a level of automation in determining system organization parameters in a wireless communication system by advantageously determining a signal propagation characterization of the system""s coverage area. This signal propagation characterization is based on measurements of a path loss-related characteristic between the system""s base stations and a plurality of wireless terminals operating throughout the coverage area. Path loss-related characteristic refers to a measurable characteristic that is partially or fully based on path loss and includes, for example, path loss, bit error rate, word error rate and frame error rate. Path loss refers to the reduction in power of a signal transmitted between two locations. Coverage area refers to the geographic area in which a wireless terminal can communicate with the base stations without substantial interruption.
In accordance with the invention, it is possible for the wireless terminals associated with subscribers located within the coverage area to provide the measurements for the path loss-related characteristic. Such a characteristic can be path loss determined by the wireless terminals measuring received signal strength (RSS) of signals transmitted at known power levels by the system""s base stations. Since the base stations are transmitting at known power levels, the path losses between the base stations and the respective locations of the measuring wireless terminals can be determined based on the differences between the known transmission powers and the RSS measurements. The determined path losses are used to form the signal propagation characterization which can be used to predict the signal strength received at locations of the measuring wireless terminals based on a corresponding increase or decrease in base station transmission power. Moreover, the individual and cumulative interference characteristics of signals transmitted on the same or adjacent communication channels between the respective base stations and the locations of the measuring wireless terminals can also be obtained from such a characterization. The measured path loss-related characteristics enable the prediction of received signal strength of transmitted signals between the base station and the wireless terminals. As a consequence, the individual and cumulative effects of signals transmitted by the respective wireless terminals at the base stations can be obtained from the measured characteristics without regard to the wireless terminals"" locations. Accordingly, the path loss-related characteristics can be measured without correlation to information regarding absolute geographic locations of the wireless terminals. Absolute geographic locations of the regions refers to the location of the wireless terminals relative to the coverage area, the systems""s base stations, or to locations outside of the coverage area.
Although some conventional systems possess the limited capability of identifying a single particular parameter setting with respect to specific mobile unit, the novel and unobvious use of the signal propagation characterization of the coverage area in this invention advantageously enables the determination of a variety of important system-based settings which can affect multiple wireless terminals. Such a signal propagation characterization can be determined from measurements taken over an extended period of time. Exemplary parameter settings that can be determined from this characterization include neighbor lists, the sets of base stations which can reuse channels and base station transmission power settings including such settings upon the addition or removal of base stations. For example, base station transmission power settings can be determined directly using the path loss characteristics. The determination of neighbor lists, and those base stations that can efficiently reuse the same or adjacent channels can be performed using isolation values derived from the predicted cumulative interference effects. Isolation values characterize the relative signal isolation of signals originating from a particular base station or its service area from interfering signals originating from another base station and service area.
Since the path loss-related characteristic measurements can be obtained during normal operation of a wireless communication system, the parameter settings can advantageously be updated automatically. Thus, the system does not require costly re-modeling and field testing as changes occur in the coverage area or system that effect the signal propagation. The invention also avoids the complex and labor intensive recording of geographic location information that is typically required during the field testing of conventional installation techniques.
Additional features and advantages of the present invention will become more readily apparent from the following detailed description and accompanying drawings.